Although frequency division multiple access was originally the favored approach in the earlier satellite communication systems, time division multiple access (hereinafter TDMA) has, in recent years, become the favored approach. The essential characteristics of a TDMA system include a plurality of geographically separated earth stations, and a repeater, usually in a quasi-stationary earth orbit. In order to effectively time divide the communication channel made available by the repeater, proper timing at the various stations is essential. To this end, a common timing mark is transmitted from the repeater, usually a replica of a signal transmitted by a selected one of the earth stations, and reception of this time marker at the various ones of the earth stations establishes a reference point in time. Because of the usual unequal distance between the repeater and the various stations, the time marker so established is relative. To obtain an absolute time standard, each of the stations corrects this time reference by actually measuring the round trip propagation delay through the repeater.
The time delay between successive receipt of the time markers represents a fixed number of bit intervals corresponding to the TDMA frame. Each of the stations employs this time delay to synchronize their own internal clock to the system clock, which is usually the clock employed at the master station.
Information received at each of the earth stations, for purposes of transmission to another one of the earth stations, can be received in a variety of forms. If the received information is analog in nature, it can quite readily be digitized synchronously with the system clock. However, where the received information is digital in form, it typically will be received asynchronously with respect to the system clock or any submultiple thereof, and therefore, some type of elastic buffering is required. Furthermore, since earth stations are a relatively expensive asset, they should be capable of processing digital information which may be received at various bit rates with only minor modifications. Finally, since the terminal will be transmitting in burst form at a relatively much higher bit rate than the received information, apparatus must be provided to, in effect, accumulate information corresponding to a complete burst, and then transmit the accumulated information at the high burst rate.
Conventionally the equipment at an earth terminal is divided into equipment which is commonly employed for the entire earth terminal and equipment which is unique to each of the various information ports. A typical example of this can be found in U.S. Pat. No. 3,838,221, wherein the common equipment includes transmit and control equipment, a multiplexer and demultiplexer, and a plurality of terrestrial interface modules (or TIMs) which are unique to each of the different information ports. Each of the TIMs performs the processes of digitizing (if necessary) elastic buffering and pulse stuffing and converting the continuous information to a high speed burst rate for transmission purposes. As a consequence, the multiplexer is capable of accepting only synchronous information at the selected burst rate from the TIMs and correspondingly, the demultiplexer makes available information to each of the TIMs at the high speed burst rate. Since cooperative action between the TIMs and the multiplexer and demultiplexer is essential, this approach requires a multiplicity of control and timing signals to be transmitted between the multiplexer, demultiplexer and TIMs, and many of these timing and control signals are at bit rates which may be as high as the burst rate. This, in effect, requires a plurality of high speed, and therefore expensive, line drivers and buffers.
A further consequence of the conventional arrangement of hardward at a TDMA earth station is a large amount of equipment duplication. More particularly, the equipment employed to change the information bit rate employs a pair of memories. In a first TDMA frame, information is written continuously into one of the memories at a bit rate the same, or nearly the same, as the rate at which the information is received. Simultaneous with writing into one of these memories, the other memory is prepared for or actually engaged in reading out the information previously written therein at a much higher rate. On the next frame the function of the memories is interchanged so that, while the first memory is read, the second is writing. These functions require an address counter for properly storing and retrieving the received information and pulse sources to operate these addressing counters. Thus, each of the TIMs employed equipment to perform this function. Since the read operation has to be synchronous with the system clock, each TIM also included a phase locked loop to assure this synchronism.
Similarly, the asynchronous TIMs employs an elastic buffer and each employs a clock source to provide a read clock for the elastic buffer. This read clock, one for each asynchronous TIM, is another example of duplicate equipment.